Had the same problem as well when sending an email to a client of mine who used Hotmail. Had had successful communications with them in the past 1.5 years then

Had the same problem as well when sending an email to a client of mine who used Hotmail. Had had successful communications with them in the past 1.5 years then suddenly, bam. Was put on the "S3140 block list". Had to use my personal Gmail to send emails to them. Never emailed another person who used Microsoft email services. Was told my email server was sending spam by them (as people had reported emails from the IP as unwanted), so I did a full security check (using logs, trying to find holes, etc), and upgraded software. No problems that I could find. After lots of back and forth, they said my IP was blocked as it was "suspicious" and they unblocked it (how kind of them). The process took a long time, which I could imagine could be devastating to a small business (luckily my client understood as I made sure to tell them how bad Microsoft is).